Field of the Invention
The present invention relates to a method of operating a fuel cell system equipped with a fuel cell for generating electrical energy by electrochemical reactions of a fuel gas supplied to an anode and an oxygen-containing gas supplied to a cathode.
Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a polymer ion exchange membrane as an electrolyte membrane. In the fuel cell, the electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between separators. The membrane electrode assembly and the separators (bipolar plates) sandwiching the membrane electrode assembly make up a power generation cell. In the fuel cell of this type, in use, typically, a predetermined number of the power generation cells are stacked together to form, e.g., an in-vehicle fuel cell stack mounted in a fuel cell electric automobile.
In the fuel cell, in order to achieve good performance in power generation reaction, it is required to maintain the solid polymer electrolyte membrane in a desired humidified state. If the water content in the electrodes is large, water clogging (flooding) may occur in the electrode catalyst and porous carbon, and if the water content in the electrodes is small, the solid polymer electrolyte membrane is dried, and the performance may become poor undesirably.
Therefore, the space in the fuel cell is kept to have a certain humidified state, and impurity ions tend to be eluted from components of the fuel cell. In particular, a large number of sulfate groups for reliably maintaining the desired ion conductivity are provided in the polymer ion exchange membrane of the membrane electrode assembly. Therefore, sulfuric acid ions may be eluted from the polymer ion exchange membrane through water in the fuel cell. When the sulfuric acid ions are retained on the catalyst surface, the power generation performance of the fuel cell may be degraded undesirably.
In an attempt to address the problem, for example, in a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2008-235093 (hereinafter referred to as conventional technique 1), a highly humidified gas is supplied to at least one of an oxidizing electrode and a fuel electrode in the fuel cell. According to the disclosure, in this manner, the electrode catalyst layer is washed with the water, and impurities are removed.
Further, in a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2013-232361 (hereinafter referred to as conventional technique 2), adjusting means for adjusting the relative humidity of the ambient atmosphere of the catalyst layer to be not more than a predetermined threshold value, prior to implementing the performance recovery process is provided. The adjusting means performs this adjustment by increasing the flow rate of an oxygen-containing gas supplied to the cathode of the fuel cell.